Fix SMD

Dear Lammps users,

I want to run constant velocity SMD simulations with fix SMD command. Using cvel mode with tether keyword does not seem to work for me as it seems that the distance between tether point and the pulling group increases (or decreases). However, I want the fix group to follow a dummy moving atom (reminiscent of AFM tip) with the help of spring. In AFM the spring length does not change too much, while in cvel with tether keyword the length increases. Thus, it does not work here.

The other option is to use cvel mode with the couple keyword. I have added a dummy atom which has a fixed velocity and would not be thermostatted and it is coupled to the SMD group. Here in the fix SMD command the velocity value is zero. And x, y and z are set to auto. However, I am not sure what is the R0. Should it be the equilibrium length of spring? I read that part of manual several time, but it is a little bit confusing.
This is the part of my lammps script related to SMD:

create_atoms 5 single 29 -0.087 -1.83
group dummy type 5
group rest subtract all dummy
neigh_modify exclude group dummy rest

fix 1 rest nvt temp 310 310 1.0
fix 3 right smd cvel 10 0 couple dummy auto auto auto 10
fix 4 left spring/self 10.0 xyz
fix 6 rest momentum 1000 linear 0 0 0 angular
fix 7 dummy move linear 1e-5 0 0 units box

Yours sincerely
Mahdi

i already responded to your inquiry.